Hours before the massacre he murdered his mother, Nancy, using the firearms she had collected.

"With hindsight, I know Adam would have killed me in a heartbeat, if he'd had the chance,” Mr Lanza said.

“The reason he shot Nancy four times was one for each of us: one for Nancy; one for him; one for (his brother) Ryan; one for me.”

He said he thinks about his son and the massacre every waking hour.

Father-of-two Mr Lanza, a vice-president for taxes at a General Electric subsidiary, said he had not seen his son for two years at the time of the shooting.

He explained Adam spent his entire life troubled by mental illness and he thought his youngest boy was an undiagnosed schizophrenic.

“You can't get any more evil. How much do I beat up on myself about the fact that he's my son? A lot,” Mr Lanza told the New Yorker magazine.

“I do not even like to say my name.”

Mr Lanza, who divorced Nancy in 2009, said he thought his son was "a normal, weird little kid" but by the time he reached middle school "it was crystal clear something was wrong."

"The social awkwardness, the uncomfortable anxiety, unable to sleep, stress, unable to concentrate, having a hard time learning, the awkward walk, reduced eye contact," he said.

"You could see the changes occurring."

Troubled by his son’s behaviour he took him to see numerous psychologists. As a result Adam was diagnosed with Asperger's syndrome - a high-functioning form of autism.

A specialist at time wrote: “While Adam likes to believe that he's completely logical, in fact, he's not at all.”

Mr Lanza, who is now remarried, said he may have overlooked troubling signs himself by accepting the Asperger's diagnosis, though he doesn't think the condition caused the violence.

"Asperger's makes people unusual, but it doesn't make people like this," he said.

He also said his ex-wife didn't detect the potential for violence.

"She never confided to her sister or best friend about being worried," he said. "She slept with her bedroom door unlocked and kept guns in the house, which she would not have done if she were frightened."

During the interview Mr Lanza refused to say where his son had been buried.

“No one knows that," he said. "And no one ever will.”

Mr Lanza gave the interview from his new home in Fairfield County, Connecticut, surrounded by crates of what he labels “the stuff”.

Since the shootings strangers have been sending Mr Lanza letters, Teddy bears and stories with titles such as “My First Christmas in Heaven”.

He said he wanted to speak out as he “wanted people to be afraid of the fact that this could happen to them.”